The purpose of this study is to test for conformational identity or differences between dissolved and crystalline carbonic anhydrase B from human erythrocytes. The results of this study will be analyzed on the basis of the three-dimensional structures of the crystalline enzyme and on results of studies on activity and chemical modifications of this enzyme in the dissolved state. The experimental approach is to react crystalline and dissolved enzyme with iodoacetate. The only histidines which will react are those accessible to solvent. Since conformational changes can easily change the reactivity of a histidyl residue, the method provides a basis for detecting such changes. The sites of reaction of 14C-iodoacetate with dissolved enzyme and 3H-iodoacetate with crystalline enzyme will be determined by preparation, purification, and characterization of peptides containing the modified histidines. Iodoacetate is an affinity labeling reagent for dissolved carbonic anhydrase B and reacts with histidine 200. The integrity of the active site of crystalline enzyme will be checked by determining the rate and specificity of the reaction with histidine 200. BIBLIOGRAPHIC REFERENCE: P.L. Whitney and H. Brandt (1976). Effects of two ionizing groups on the active site of human carbonic anhydrase B.J. Biol. Chem. (In press).